Hi all, quick question, I don't know if it's standard for vfr's but for no reason my revs jump from idle (1.2k/1.8k) to 3k or there abouts for no reason and sits there for 5 seconds or a minute or two, sometimes a bit more then goes back to idle, unable to identify any conditions that are similar (other than when the bikes been running for a while) that might point to a reason, does anyone have experience of this or have any ideas please? Vfr750fp. Thanks in advance.
Thanks all, carbs is what I thought but thought I'll ask the experts (only had it since beginning of August)
I am having the same problem with my '83, yesterday I was balancing carbs with my new Carbtune Pro, it was idling around 1200, then suddenly it jumped to 3000, like some flicked a switch. Going to have looked at by local mechanic and will post resolution.
There is probably an air leak or two on the carb boots. The carb boots get hard and the bands loosen up. Then they suck air. How to check. I have done this on my 70's CB inline SOHC bikes. I get a spray bottle with clean water and with the bike running spray each carb boot separately with the water. If there is and air leak you will see a change in the idle speed. Two solutions. the easy one tighten the carb boot bands. It might work it might not depending on how hard the boots are. Buy new boots and replace. Since I don't know these bikes well I hope there are carb boots and I don't sound like a DA.
I'm with you on the spraying the carb boots to detect air leaks, but is water really the BEST thing to use? I'm thinking maybe some Chemtool or starting fluid......
I have never had an issue using water. There is so little water being sucked in that it doesn't hurt anything and it leaves no residue. I know some folks use like WD40 or something else. I just prefer water.
If the engine has a great deal of carbon, this water will knock the carbon right off of the chamber. I knew this was done a lot in the old days for that purpose. I did some googling when it came up, and got some stories which surprised me, such as how some added a water bottle injection system to the vehicle: https://www.physicsforums.com/threads/decarbonizing-an-engine-with-water-myth-or-fact.793344/ The atmosphere already has water in it, so you're not really doing anything different, unless you give it a hydrolock-enabling volume of water.
Whenever i clean carbs for members here i usually ask them to include the carb boots for my inspection. Often, very often i see cuts and cracks (which could leak air) caused by rough installation methods or age-related deterioration, so maybe the same in your case ?? Or maybe a plug is sparking intermittently leading to the inconsistent idle speed.
I needed to listen to myself closer, I agree vacuum leaks would cause rpm problems, but not the kind I had. Not a sudden jump. I found to problem to be our old friend corroded connectors, shame on me. I had set idle to about 1200, then suddenly jump to 3000. Put it back together the other day, then I thought about what I said, "like a switch", ah ha, electrical problem. Took it apart again today, (moved tank and removed airbox), and started it again. Started at 3000 this time, then I started wiggling wires to the coils, bang, it dropped to 1200, wiggled some more and back to 3000! Took coils out and sanded male pins till bright brass, and then used emory board to burnish inside of female connectors. Bingo, all is well. Used CarbTune Pro to balance carbs and put it back together for the tenth time. Winter project will be to go through every connector to burnish and clean, for now, going for a ride!
Didn't some goofball suggest it might be electrical, i.e., sparkplug firing intermittently ?? I doubt your carbs were far off synchwise.
Not too far off, but that was after my first attempt when it wasn't running right. Just came back from getting gas and a short ride, running great. Engine ran strong 1983 to 1990, then it sat till 2001, got it running, but I was busy till 2015, still running strong with fresh Amsoil Synthetic. I won't let it sit another 15 years!
